The invention relates to reducing the effect of noise during image capture operations in a digital imaging system.
Referring to FIG. 1, a common digital image processing system includes camera 100, computer 102 coupled to camera 100 by communication link 104, and display unit 106. Computer system 102 may be a personal computer (PC). Communication link 104 may be a serial bus such as the universal serial bus (USB). Display unit 106 may be any convenient display device such as a cathode ray tube (CRT) or liquid crystal display (LCD).
Camera 100 may use imager 108 to generate an electrical representation of optical image 110. To accomplish this, imager 108 may include a sensor having an array of photon sensing elements. During an integration time or interval, each sensor element accumulates light energy from that portion of optical image 110 that is focused on it by camera 100 optics (not shown in FIG. 1). At the expiration of the integration interval, sensor elements indicate the intensity of the received light energy by, for example, an analog voltage signal. Camera 100 typically processes the indications from sensor elements to form a frame of digital data which may then be stored in memory internal to camera 100 (not shown in FIG. 1), and/or transferred to computer 102.
Typically, the frame of data does not indicate an exact duplicate of optical image 110 due to imperfections introduced by camera 100. As examples, camera 100 may introduce optical distortion and noise. One type of noise may be dark current noise. Dark current noise may be defined as sensor element current that is present even when the sensor element is not illuminated. Dark current noise tends to reduce the captured image's dynamic range and signal to noise ratio (SNR) and places a limit on sensor element integration time. Dark current also tends to increase as the temperature increases, rendering many cameras substantially unusable in high temperature environments. Another type of noise is offset noise. Offset noise may manifest itself as increased signal values (e.g., voltage or analog to digital conversion counts) which do not represent a feature of the captured image. Offset noise may be introduced by signal amplification and analog to digital conversion circuitry.
Some cameras attempt to cancel out dark current noise from a sensor element by subtracting a predetermined noise level from the intensity that is indicated by the sensor element. The predetermined noise level may be determined from, for example, an extra sensor element that is not exposed to light during image capture operations. Similarly, some cameras attempt to compensate for offset noise by subtracting a predetermined value from analog to digital converter output. Neither of these methods correct for actual dark current noise or offsets in a manner that improves the dynamic range of the captured images.
Thus, it would be beneficial to provide a mechanism to automatically and accurately compensate for the noise introduced by a digital camera during image capture operations.